12.11: Exceptions and ensure, ifCurtailed Interaction

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Now that we saw how exceptions work, we present the interplay between exceptions and the ensure: and ifCurtailed: semantics. Exception handlers are executed then ensure: or ifCurtailed: blocks are executed. ensure: argument is always executed while ifCurtailed: argument is only executed when its receiver execution led to an unwound stack.

The following example shows such behavior. It prints: should show first error followed by then should show curtailed and returns 4.

[[ 1/0 ]
    ifCurtailed: [ Transcript show: 'then should show curtailed'; cr. 6 ]]
    on: Error do: [ :e |
        Transcript show: 'should show first error'; cr.
        e return: 4 ].

First the [1/0] raises a division by zero error. This error is handled by the exception handler. It prints the first message. Then it returns the value 4 and since the receiver raised an error, the argument of the ifCurtailed: message is evaluated: it prints the second message. Note that ifCurtailed: does not change the return value expressed by the error handler or the ifCurtailed: argument.

The following expression shows that when the stack is not unwound the expression value is simply returned and none of the handlers are executed. 1 is returned.

[[ 1 ]
        ifCurtailed: [ Transcript show: 'curtailed'; cr. 6 "does not display it" ]]
    on: Error do: [ :e |
        Transcript show: 'error'; cr. "does not display it"
        e return: 4 ].

ifCurtailed: is a watchdog that reacts to abnormal stack behavior. For example, if we add a return statement in the receiver of the previous expression, the argument of the ifCurtailed: message is raised. Indeed the return statement is invalid since it is not defined in a method.

[[ ^ 1 ]
        ifCurtailed: [ Transcript show: 'only shows curtailed'; cr. ]]
    on: Error do: [ :e |
        Transcript show: 'error 2'; cr. "does not display it"
        e return: 4 ].

The following example shows that ensure: is executed systematically, even when no error is raised. Here the message should show ensure is displayed and 1 is returned as a value.

[[ 1 ]
        ensure: [ Transcript show: 'should show ensure'; cr. 6 ]]
    on: Error do: [ :e |
        Transcript show: 'error'; cr. "does not display it"
        e return: 4 ].

The following expression shows that when an error occurs the handler associated with the error is executed before the ensure: argument. Here the expression prints should show error first, then then should show ensure and it returns 4.

[[ 1/0 ]
        ensure: [ Transcript show: 'then should show ensure'; cr. 6 ]]
    on: Error do: [ :e |
        Transcript show: 'should show error first'; cr.
        e return: 4 ].

Finally the last expression shows that errors are executed one by one from the closest to the farthest from the error, then the ensure: argument. Here error1, then error2, and then then should show ensure are displayed.

[[[ 1/0 ] ensure: [ Transcript show: 'then should show ensure'; cr. 6 ]]
    on: Error do: [ :e|
        Transcript show: 'error 1'; cr.
        e pass ]] on: Error do: [ :e |
            Transcript show: 'error 2'; cr. e return: 4 ].